Most of the materials presently used to coat naval platforms and to encapsulate acoustic sensors have been around for decades. The performance of these materials from a variety of perspectives is truthfully characterized as “barely adequate.” Because of these limitations, design engineers have had to accept many compromises concerning cost and/or service lifetime. These compromises now threaten the viability of some of the U.S. Navy's most cherished future hardware concepts, such as miniaturized, distributed sensors, large area sensors smart skins, and hi-powered acoustic sources. They are also inconsistent with the Navy's current “total ownership cost reduction” thrusts in the areas of service lifetime extension and reduced maintenance requirements. Using existing materials alone many of these advanced concepts and reliability improvements simply cannot be realized. The existing materials are used because of their ease of application and because other concerns and material requirements (primarily acoustical) are viewed as more important than their barrier properties.
The encapsulants used in acoustic applications must be acoustically clear. The term “acoustically clear” means that acoustic energy is able to enter and transit through the material with a minimal amount of reflection, loss, distortion or absorption. Only a small set of polymers have been found to possess the physical properties and chemical structures that ensure acoustic clarity. Of these materials, those that are castable, such as polyurethane, tend to exhibit greater water permeability than those that are vulcanizates, such as butyl rubber, EPDM (ethylene propylene diene monomer) rubber, and polychloroprene rubber.
Castable materials are preferred because they can be poured into molds and cured at room temperature or at moderate temperatures in an oven. Commonly used acoustic devices include sensors and sources. These devices are made from materials such as piezoelectric crystals and polymers, are temperature sensitive, and cannot be subjected to high temperatures and pressures. The vulcanizates require higher temperatures and pressures to cure. Thus they are typically made in the form of a boot or covering that is then adhesively bonded or mechanically clamped to the acoustic device. Modification of castable, acoustically clear materials to make them less permeable to water is highly desirable. Any such modifications would have to preserve the superior acoustic properties of such materials while at the same time, greatly enhancing their barrier properties.
Nanomaterials and polymer nanocomposite technology might be able to enhance current encapsulants. As its name implies, a nanocomposite contains particles with at least one nanoscale (10−9 meter) aspect (length, width or thickness). Because of the enormous surface area a dispersion of such particulates possesses, relatively small loadings (typically a few weight percent) in a suitable polymer matrix may exhibit orders of magnitude-scale improvements in certain physical properties and/or influence the structure of the polymer matrix in ways not possible to achieve with conventional technology. Careful selection of the chemistry and geometry of the nanoparticles frequently allows the bulk properties of the resulting polymer nanocomposite to be close to those of the unfilled polymer matrix: while greatly enhancing a specifically targeted physical property of interest. Such “input/output” selectivity promises to deliver significantly improved coatings and encapsulants for naval applications including coatings with orders of magnitude lower water/gas permeability and encapsulants with ten times the normal polymer thermal conductivity.
The barrier-property-enhancing fillers are nanoscale (ca. 3-10 nanometers thick by several hundred to several thousand nanometers across) plates derived from a variety of different phyllosilicate clay minerals, such as montmorillonite, hectorite, saponite, bentonite and the like. These materials are known as “sheet silicates” because they are made up of tiny particles which are themselves composed of a large number of extremely thin mineral sheets (like mica). Thousands of these individual sheets stacked on top of each other form an individual clay mineral particle. The sheets are only loosely held together in the vertical direction by Van der Waals forces. Thus, the particles are permeable in the X-Y direction (between sheets), but they are essentially impermeable in the Z direction (through the sheets). Clay minerals are preferred as starting materials because they are composed of nano-to-micron scale particles that can be converted (with the proper chemical pre-treatment) into large numbers of individual sheets/plates with large aspect ratios (typically 100:1 or greater).
These fillers are not typically used in acoustic applications because they are not acoustically transparent. The speed of sound, c, in the composite is approximately equal to:
                    c        =                              M            ρ                                              (        1        )            where:M=modulus of elasticity; andρ=density.For acoustic clarity, the product of sound speed, c, and density, ρ, of the coating/encapsulant must be close to the ρc product of the surrounding medium, seawater. Unfilled polyurethane has a ρc product approximately equal to that of seawater. Adding a filler of higher density, like clay, causes the ρc product of the resulting composite to deviate from the ρc product of seawater. The more filler, the higher the density. Also, as filler is added, the modulus increases, and thus, so does the sound speed, c. In conventional composites it is common to add 20-30% by weight of filler. This makes the composite material no longer acoustically transparent. Thus, the use of fillers in polyurethane has always presented a problem.
There are three possible particle-matrix in clay particulate-based polymer nanocomposites shown in FIGS. 1A, 1B and 1C. First, in FIG. 1A, the composite 10A is shown with the clay particles 12 dispersed within the polymer matrix 14 in their natural state. This geometry does not lead to especially interesting or useful properties because the clay particles 12 are porous and do not present an obstacle to liquid travel. FIG. 1B shows the “exfoliated” or “delaminated” geometry as 10B. In this geometry, the individual sheets 16 comprising each clay particle are separated from each other and dispersed individually within the polymer matrix 14. Sheets 16 are disposed randomly in the matrix. Because the individual sheets 16 are not overlapped, they do not present significant barriers to fluid travel. The third polymer-particulate geometry is shown in FIG. 1C. The geometry of sample 10C is referred to as “intercalated.” In this arrangement, a single layer of polymer chains 18 is infiltrated between the individual sheets/layers 20 that comprise a clay particle. A polymer matrix 14 is formed outside of the intercalated particles. This geometry leads to alternating, thin layers of silicate and polymer a few nanometers apart.
Both the exfoliated and the intercalated geometries lead to improvements in the barrier properties (including a significant decrease in water permeability) of the resulting nanocomposite; however, the intercalated geometry leads to significantly better properties. The primary difference between creation of the geometries is the time and extent of mixing or sonicating. As mixing increases, the clay particles become delaminated and are more likely to form the exfoliated geometry.
For applications in which water permeation is a critical concern, hydrophobic, non-polar polymers such as EPDM and butyl rubber are typically used. These materials are vulcanizates which are crosslinked through the use of heat in pressurized molds. These materials must be molded first, and then bonded to the sensor. Their non-polar nature makes it difficult to bond anything else strongly to them. Thus, most EPDM and butyl rubber boots are secured to the underlying sensor (where possible) by metal bands or other mechanical means. What marine sensor designers would really like to have is an acoustically “clear” encapsulants that will cure in place where it is poured, and which, when cured, will exhibit very low water permeability constants similar to (or better than) those of EPDM and butyl rubber. At the present time, no such materials exist.